If you're doing low volume stuff, I wouldn't worry about harming the speaker. [disclaimer] I bear no responsibility should the speaker blow [/disclaimer]

As far as the ohm thing. There are about 6.2 billion people in the world who know more about this stuff than I do. In short though, ohm ratings and allowances are kind of like a step-down type system in reverse. You can run (1) 4 ohm cab out of your 4 ohm head and it'll run at 200 watts. You can run( 1) 8 ohm cab and it'll put out around 120 watts. 16 ohm, 80 watts or so. That being said, you can plug (2) 8 ohms's or (4) 16 ohms to take advantage of the whole 200 watts. Get it? BTW, it would probably bad to run (2) 4 ohm cabs out of your amp.